1. Field of the Invention
The present invention relates to an ink jet recording apparatus and ink jet recording method that carry out recording by discharging ink to a recording medium, and more particularly to reduction of density unevenness such as so-called bandings at the boundaries between recording scanning areas.
2. Description of Related Art
Recording apparatuses employed as print output means of pictures in printers, copying machines and facsimile machines, or those used as print output equipment of complex electronic machines or workstations incorporating a computer or word processor are constructed such that they record pictures on a recording material (also called recording medium hereinafter) such as paper or a plastic sheet in response to image information (including all output information including text information). Such recording apparatuses can be divided into ink jet printing systems, wire matrix printing systems, thermal printing systems, laser beam printing systems and the like. Among these, the recording apparatus based on the ink jet printing (called ink jet recording apparatus hereinafter) carries out recording by discharging ink onto a recording material from a recording means including recording heads, and has such advantages over the other recording systems that it can easily implement high resolution, high speed printing, and is quiet and inexpensive. On the other hand, to meet the strong demand for color output such as color pictures, many types of color ink jet recording apparatuses have been developed recently.
It is common for such an ink jet recording apparatus to employ a recording head that integrates a plurality of ink discharge orifices and ink passages as a recording unit (called multi-head hereinafter) that integrates multiple recording elements arranged, and to comprise a plurality of multi-heads to deal with the color printing.
FIG. 1 is a schematic perspective view showing a major portion of a system for carrying out recording (also called printing hereinafter) on paper using the multi-head. In FIG. 1, each reference numeral 101 designates an ink jet cartridge. The ink jet cartridges 101 include ink tanks for holding four color inks, black, cyan, magenta and yellow, and a multi-head 102 corresponding to the inks. FIG. 2 is a schematic diagram illustrating discharge orifices (also called nozzles hereinafter) disposed in the multi-head 102, which are seen from the z direction in FIG. 1. In FIG. 2, the reference numeral 201 designates n nozzles arranged at a pixel density of N per inch (N dpi) in the multi-head 102.
Returning to FIG. 1, the reference numeral 103 designates a sheet transfer roller that rotates in the direction indicated by an arrow with holding printing paper P between it and an auxiliary roller 104 to transport the printing paper P in the y direction. The reference numeral 105 designates a pair of feed rollers for feeding the printing paper. The pair of rollers 105 also rotates with holding the printing paper P as the rollers 103 and 104, and their rotation speed is set less than that of the sheet transfer roller 103 to exert tension on the printing paper. The reference numeral 106 designates a carriage for supporting the ink jet cartridges 101 for causing them to scan and print. The carriage 106 stays at a home position h denoted by broken lines in this figure while it is free from printing or it is carrying out recovery processing or the like of the multi-head 102.
The carriage 106 which is placed at the home position h before the start of printing moves in the x direction when a print start command is issued, and carries out printing on the paper with a height of n/N using the n nozzles 201 disposed on the multi-head 102 at a density of N per inch. Completing printing to the opposite end of the paper, the carriage returns to the home position, and moves again in the x direction to continue printing. Before starting the second printing after the end of the first printing, the sheet transfer roller 103 rotates in the arrow direction so that the paper is transported in the y direction by an amount of n/N inch. Thus, by repeating such printing by the multi-head 102 and paper transport by an amount of n/N inch per main scanning of the carriage 106, printing of one page, for example, can be completed. Such a printing process is called one pass print mode below.
The one pass print mode is most suitable for printing text or graphical images at a high speed as with a monochromatic printer. Its paper transfer system operation, however, sometimes involves a slight error in general. FIGS. 3A to 3C show examples of such paper transfer errors, in which FIG. 3A illustrates an ideal paper transfer, whereas FIG. 3B illustrates a space of a height s at a discontinuous joint between dots of Kth and (K+1)th scannings, and FIG. 3C illustrates an overlap of a height s. The space of the height s due to the paper transfer error brings about a white banding of the height s in the main scanning direction in a printed picture, and the picture is considered as a defective one. In contrast with this, since the joint is kept continuous in the overlap case of the height s, it is often unperceived, and the picture is not handled as a defective picture. Thus, to compensate for the paper transfer error visually, the paper transfer amount of a conventional paper transfer system is sometimes set less than a standard value so that the joint is kept continuous.
On the other hand, to print a graphical image, various recording characteristics such as uniformity of color development, gray levels and uniformity of density are required. In particular, with regard to the uniformity of the density, it is known that slight difference in individual nozzles, which are produced in a multi-head fabrication process, bring about difference in the volume or direction of ink discharged from the nozzles, and this causes density unevenness of the printed image, thereby degrading its quality.
An example of this will be described with reference to FIGS. 4A to 4C and FIGS. 5A to 5C. In FIG. 4A, the reference numeral 41 designates a multi-head with eight nozzles 42. Each reference numeral 43 designates an ink droplet discharged from one of the nozzles 42. Ideally, it is desirable that the ink be discharged in the same direction in about the same volume as shown in this figure, which would bring about uniform dots on a sheet as illustrated in FIG. 4B, and produce an image without density unevenness as illustrated in FIG. 4C.
In reality, however, the nozzles have errors as described above, and hence if printing is carried out in the same manner as shown in FIGS. 4A to 4C, the volume and direction of the ink droplets discharged from the nozzles become somewhat random as illustrated in FIG. 5A. As a result, dots are formed on a sheet as illustrated in FIG. 5B. In this example, there arise in the main scanning direction of the head cyclic blanks that cannot satisfy the area factor of 100%, or black bandings due to unsuitable overlaps of dots, or white bandings as illustrated in the center of FIG. 5B. The image consisting of such dots has a density distribution as illustrated in FIG. 5C in the nozzle arrangement direction, and hence a density unevenness is perceived.
As countermeasures against such density unevenness or bandings, Japanese patent application laid-open No. 60-107975/1985, for example, discloses the following method in a monochromatic ink jet recording apparatus. The method will be described briefly with reference to FIGS. 6A to 6C and 7A to 7C.
Although this method carries out three scannings of the multi-head 41 as illustrated in FIG. 6A to complete the printed area as shown in FIG. 6B, half of the printed area corresponding to four nozzles is completed in two scannings (also called passes hereinafter). More specifically, the eight nozzles of the multi-head 41 are divided into two groups consisting of the upper four nozzles and lower four nozzles, and dots that are printed by one group of the nozzles (namely, four nozzles) in one main scanning produce from given image data an approximately half decimated or thinned out image with a predetermined pattern. Then, the second scanning completes the area corresponding to the four remaining nozzles by carrying out the printing in accordance with the prescribed patterns. Such a print mode is referred to as a decimated multipass print mode hereinafter.
Using such a print mode will halve the effect of the characteristics of each nozzle on the printed image in the scanning areas, even if the same multi-head as shown in FIGS. 5A to 5C is used. Thus, the printed image becomes as shown in FIG. 6B, in which the black bandings or white bandings are less conspicuous than those in FIG. 5B. As a result, the density unevenness is reduced considerably as shown in FIG. 6C as compared with that of FIG. 5C.
In such a recording scheme, the image data is usually divided into the first and second scannings in accordance with predetermined arrangements that are complementary to each other. It is most common that the image data arrangements (decimated pattern) are staggered check arrangements of respective pixels in rows and columns as shown in FIGS. 7A to 7C. Accordingly, to record a unit printed area (an area corresponding to the four nozzles), the printing is completed by the first and second scannings that carry out printing in a staggered check arrangement and in a counter-staggered check arrangement, respectively.
FIGS. 7A, 7B and 7C illustrate a process in which an area is recorded using the staggered check and counter-staggered check patterns.
In FIGS. 7A to 7C, the first scanning carries out the recording of the staggered check pattern using the lower four nozzles (FIG. 7A). After incrementing the paper by an amount of four pixels (half the height of the head), the second scanning carries out recording in the counter-staggered check pattern using all the eight nozzles (FIG. 7B). After incrementing the paper by an amount of four pixels (half the height of the head), the third scanning carries out recording in the staggered check pattern again (FIG. 7C). Thus, alternating the staggered and counter-staggered check patterns after incrementing the paper by an amount of four pixels can complete the four-pixel high recorded area for each scanning.
As described above, high quality image with little density unevenness can be obtained by completing the printing using two different nozzles in the same area.
Thus, when outputting monochromatic texts or graphical images at a high speed, the one pass print mode is executed in which the paper transfer amount is set less than a standard value, whereas when outputting high quality color graphical images, the decimated multipass print mode is carried out to improve the problem of density unevenness at joints between the scanning areas or in other areas.
However, increasing the pixel density and gray levels of recording to produce picture-like high quality color graphical image, for example, will make the black bandings, which are rather visually inconspicuous conventionally, increasingly noticeable at the scanning boundaries in the decimated multipass print mode, thereby degrading the image quality.
One of the reasons for this is that although printing of an image area other than boundaries of each scanning area is completed in a time period corresponding to the number of scannings necessary for completing the image, an area adjacent to a boundary (also called boundary region hereinafter) is affected by the printing of the first scanning of the next scanning area, so that it takes extra time corresponding one scanning to finally determine the density at the boundary region. Generally, dots that constitute individual pixels are formed with sizes greater than the size of the pixels, and hence the dots of the adjacent pixels overlap in part on each other. Accordingly, with regard to the pixels in the boundary regions, their density is determined after the printing of the next scanning area is completed.
In the foregoing case, however, the dots in the boundary region between the two adjacent scanning areas are formed on a recording sheet with a time delay corresponding to one scanning, and this brings about a black banding resulting from density unevenness due to the difference of penetration and fixation of ink that produces dots on the recording medium.
FIGS. 8A to 8D are schematic diagrams illustrating states of landing and fixation of ink on a recording sheet, which indicate that the fixation degree of the previously landing ink affects the fixation of the next landing ink.
Although the previously landing ink brings about a state as denoted by black portions in these figures when it is sufficiently fixed, if its fixation is insufficient, the subsequently landing ink may penetrate in part under the previously landed ink as indicated by shadowed portions in these figures. If such a dot overlap due to the time delay takes place in the boundary region, it can appear as a banding distinct from the remaining portion. In particular, since such overlaps between the adjacent dots take place frequently in solidly printed areas with high printing duty, they will bring about increasing black bandings due to conspicuous density unevenness. Furthermore, setting the paper transfer amount less than the standard value to establish compatibility with the one pass print mode will cause more overlaps of the adjacent dots, resulting in more noticeable black bandings.